(1) Field of the Invention
The invention relates to the general field of semiconductor integrated circuits, more particularly to circuits that may be personalized, repaired or modified by means of fusible links.
(2) Description of the Prior Art
Semiconductor integrated circuits (ICs) that have optimum density and/or performance cannot, in general, be repaired or modified. There exists, however, a large class of ICs that are intended to be repairable and/or modifiable. In certain cases, no real circuit exists until the IC has been personalized by breaking certain connections, thereby determining how the components are to be connected to one another.
One method for realizing circuits of this type is to arrange for some of the connections between components to be capable of being permanently removed (opened) as desired. The portion of such a connection that is actually physically removed is referred to as a fusible link.
In general, the method for removing a particular fusible link comprises heating it briefly, but with sufficient intensity so that it vaporizes without appreciably heating other circuitry in its vicinity. Delivery of the heat pulse that is required to produce the selective vaporization of any particular fusible link is achieved in one of two ways. An X-Y addressing scheme may be used to deliver a high current pulse to the link so that vaporization occurs as a result of Joule heating or a high energy beam of intense laser light may be directed at the surface of the fusible link for a short time.
A common problem, associated with both methods of vaporizing fusible links, is that some, or all, of the debris that is a byproduct of said vaporization process recondenses on the surface of the IC and may cause short circuiting. This is commonly dealt with by coating the entire integrated circuit with a layer of insulation as a final step in the manufacturing process, thereby electrically isolating it from any conductive material that may recondense on it. Said final layer of insulation also covers the fusible links, thereby increasing the mass of material that must be vaporized whenever a particular link is to be blown.
Ideally, all the heat energy that is directed at a particular fusible link will be used for effecting its vaporization. In practice, some of this energy will be conducted into the substrate, or main body of the integrated circuit, away from the fusible link. Thus it will not be available for the vaporization process and, additionally, it may have an undesirable effect on the integrated circuit itself. This phenomenon can lead to a narrowing of the process window that is available for heating the link--too little energy and vaporization of the link is incomplete, too much energy and surrounding circuitry gets damaged.
A number of issued patents address various aspects of these two problems. Takagama (U.S. Pat. No. 4,536,949 August 1985) is concerned with electrical (as opposed to laser) fusing. The fusible link sits at the bottom of a deep trench on whose walls the products of vaporization are expected to condense, thereby keeping them away from other parts of the integrated circuit. Billig et al. (U.S. Pat. No. 5,025,300 June 1991) is concerned with laser fused links and is similar to Takagama in that the link lies at the bottom of a trench. Unlike Takagama, Billig also makes use of a final protective layer of insulation.
Monotami et al. (U.S. Pat. No. 5,241,212 August 1993) also place the fuse in a trench but the protective layer stops at the trench'stop edge, thereby leaving the link itself exposed. The upper surface of the fusible link is level with the bottom of the trench. In an alternative, optional, embodiment, a layer of insulation 6-8,000 Angstrom units thick is deposited over the fuse. The fuse is heated through laser energy, most of which passes through this optional layer.
An example of a fusible link structure of the type found in the prior art is shown in FIG. 1 as a schematic cross-section. The fusible link (layers 3 and 4) lies on silicon dioxide layer 2 which has been formed on the surface of silicon substrate 1 which comprises the integrated circuit. The fusible link has been overcoated with passivation layers 5 and 6.
Experiments on fusible link structures such as the one illustrated in FIG. 1 have shown that the range over which the applied laser energy may vary is quite narrow. For example, as shown in FIG. 3, over an energy range of from 0 to 2 microjules, the minimum energy required to cause links to open up was found to be about 0.5 microjoules. However, between 0.5 and 1 microjoule, the resistance of links that had been subjected to laser pulses was found to vary over a wide range, from short to open circuits. Between 1 and 1.5 microjoules, links that had been subjected to laser pulses were consistently found to have open circuited, as intended. However, in the range of from 1.5 to 2 microjoules, a wide variation in link resistance, similar to what was seen for the 0.5 to 1 microjoule range, was again seen. In the latter case, the cause was identified as being the result of heat reaching the underlying silicon substrate in amounts sufficient to melt some of the silicon, which then contributed to the recondensed debris.
It should also be noted that, for laser heated links in general, the duration of the laser pulse will always be slightly longer than the minimum time needed to cause the link to explode. This is inevitable, given that the exact energy needed varies slightly from link to link. As a consequence, the underlying material on which the link rested prior to its explosion will be directly exposed to the laser for a short time.